Controller and operation method thereof

A controller including: an initialization unit initializing values and states of variable nodes and initializing values of check nodes; a variable node update unit updating the values and states of the variable nodes; a check node update unit updating the values of the check nodes based on the updated values and states of the variable nodes; and a syndrome check unit deciding iteration of the operation of the variable node update unit and the check node update unit when the values of the check nodes are not all in a satisfied state, the variable node update unit calculates reliability values of the variable nodes and a reference flip value based on a result of a previous iteration, and the variable node update unit updates the values and states of the variable nodes based on the reference flip value and the reliability values and states of the variable nodes.